This invention relates to electronic amplifiers, and more particularly to switched-mode radio frequency (RF) power amplifiers.
Transmitters in battery-powered devices need to be efficient so that battery energy is conserved. The operating time of modern battery-powered communication devices is to a large extent limited by the power consumption, a significant part of which is attributable to the power amplifier (PA). It is therefore of interest to keep the PA efficiency as high as possible.
In conventional radio transmitters, the signal information is often represented as two channels in quadrature phase that can be mixed together to form a combined low-power signal that is amplified for transmission. A problem arises in that the amplifier types having the highest potential efficiency are also highly nonlinear, e.g., the switching types Class D, E, and F. Amplifier nonlinearity is not a problem with prior cellular telephony devices that employ constant-envelope (amplitude) modulations, but as the data rates of modern communication systems increase, more spectrally efficient types of modulation that modulate both the phase and envelope of the carrier signal are now being used.
Transmitters in many modern communication systems, such as cellular radio systems having carrier frequencies of 1-2 gigahertz (GHz) or so, need to have wide bandwidth, wide dynamic range, and high accuracy (low distortion) in phase and envelope. In addition, it is currently preferable that high-performance amplifiers be implemented in CMOS for reasons of cost and integration.
To enable a Class D amplifier, for example, to handle signals with non-constant envelopes, the amplifier can use a form of pulse width modulator (PWM) for linearization, such as described in F. Raab, “Radio Frequency Pulsewidth Modulation”, IEEE Trans. Comm. pp. 958-966 (August 1973); M. Nielsen and T. Larsen, “An RF Pulse Width Modulator for Switch-Mode Power Amplification of Varying Envelope Signals”, Silicon Monolithic Integrated Circuits in RF Systems, pp. 277-280, Aalborg University (2007); and International Publication WO 2008/002225 A1 by H. Sjöland, for example.
A radio transmitter combining two or more outputs can use PWM in several different ways, but the basic concept used to pulse-width modulate an RF signal is much the same as for a low-frequency Class D amplifier employing PWM. One difference is that instead of low-pass filtering the output signal to extract information at the same frequency as the input signal to an amplifier, a band-pass filter (BPF) is used in a transmitter to extract information around the PWM switching frequency. This is sometimes called band-pass PWM (BP-PWM) or RF PWM.
As described in WO 2008/002225, FIG. 1 is a block diagram of a portion of an RF transmitter that includes a switched-mode power amplifier 10, an output band-pass filter BPF, and an antenna 12. The amplifier 10 receives an input envelope signal input El that is connected to a first input of an arithmetic subtractor SUB. The output of the subtracting unit SUB is provided to an amplifier Av, whose output is provided to a pulse-width modulator PWM that also receives an RF carrier signal C that is to be provided with phase-information content and transmitted. The output of the modulator PWM is provided to a power amplifier PA that receives a supply voltage Vdd and provides an amplified version of the output of the modulator PWM to the bandpass filter BPF, which is connected to the antenna 12. A second input of the subtractor SUB receives a feedback signal from the output of the power amplifier PA. The feedback signal is produced by a low-pass filter LPF that is connected to the output of the power amplifier PA. The output of the filter LPF is digitized by a first analog-to-digital (A/D) converter A/D1 and provided to a digital signal processor DSP. The supply voltage Vdd is digitized by a second A/D converter A/D2 and provided to the processor DSP, which is suitably configured to produce the feedback signal that is converted from digital form to analog form by a D/A converter D/A and provided to the subtractor SUB.
An RF switched-mode PA having an input rectangular wave produces its maximum output power when the duty cycle is 50% (after band-pass filtering to extract the fundamental frequency), which is to say that the PA's maximum output power is generated when the input signal is a square wave. To reduce the PA's power/envelope, the duty cycle is altered with BP-PWM, effectively reducing the pulse-widths. This is illustrated by FIG. 2, which shows typical input and output signals for maximum output, i.e., 50% duty cycle (FIG. 2A) and a reduced duty cycle (FIG. 2B) that results in a smaller output signal envelope.
FIG. 3 depicts an example of a Class D amplifier stage 300 that can be used with BP PWM, receiving an input switched signal, such as the square wave shown in FIG. 2A. The amplifier 300 operates as an inverter stage, with high-power transistors 302, 304 and a band-pass filter 306 to extract the fundamental frequency of the amplifier output signal that is provided to a resistive load RL. For a square-wave input signal, the amplifier 300 would produce an output signal similar to that shown in FIG. 2A. As depicted in FIG. 3, the filter 306 includes an inductance 308 and a capacitance 310, but of course other topologies can be used. In addition, the amplifier 300 could be configured as a Class E or Class F amplifier.
In general, a switched-mode amplifier is most efficient when it operates at or close to its peak output power. This is also true for BP-PWM described above. Losses due to parasitic capacitances occur at each edge of the input (switched) signal, and in the case of an inverter-type output stage like amplifier 300, there is also a short period for every switching transition during which a short-circuit current flows through the amplifier transistors (unless the amplifier is configured to have a dead-time period).
For a constant number of input pulses per unit time, the switching losses are (approximately) constant for all amplifier output power levels, and so efficiency can decrease quickly as the input signal decreases. FIG. 4 illustrates the efficiency problem. FIG. 4A is a plot of simulated amplifier efficiency against output power, roughly illustrating how the efficiency usually behaves. FIG. 4B is a plot of the probability density of output power against output power for an illustrative amplitude-modulated (AM) signal.
With modern spectrally efficient modulations, the average signal power is in many cases significantly less than the maximum instantaneous output power. In other words, the power amplifier operates most of the time at fairly low power levels and not close to its maximum where it is most efficient. Overcoming such behavior and other aspects of switched-mode amplifiers with PWM are thus highly desirable.
One known approach to improving the efficiency of amplifiers with PWM at less than maximum output power is a Doherty amplifier, which is described in the literature, e.g., V. Viswanathan, “Efficiency Enhancement of Base Station Power Amplifiers Using Doherty Technique”, M. Sc. Thesis, Virginia Polytechnic Institute and State University, 2004. Nevertheless, the gain and delay of the component amplifiers of a Doherty amplifier can be hard to match, and combining the powers of the component amplifiers and matching to a load can be hard to do accurately.